1. Field of the Invention
The present invention relates generally to a signal converting circuit and, more particularly, is directed to a signal converting circuit for converting a single-ended input signal into a differential output signal.
2. Description of the Prior Art
Signal converting circuits, for example, of the differential amplifier type, for converting a single-ended input signal into a differential output signal are well-known in the art. One such signal converting circuit includes a pair of transistors which form a differential amplifier. A DC bias is applied to the bases of the transistors and one of the transistor bases is grounded to eliminate any effect from alternating current. Further, a signal voltage is applied to the base of the non-grounded transistor and a differential output is derived from the collectors of the two transistors. However, since a bias circuit is required with such differential amplifier, the circuit construction becomes relatively complicated and the voltage source for the transistors cannot be effectively utilized.
One known circuit that has attempted to eliminate the problems in the above-described circuit so as to effectively utilize the voltage source without using a bias circuit, includes a first transistor having its base connected to ground through a pair of series diodes through which a constant current flows. A second transistor has its base connected to the emitter of the first transistor and to ground through a diode, and is further supplied thereat with a signal current. With this circuit, differential output currents are generated at the collectors of the two transistors. However, in order for this circuit to operate properly, the constant current supplied to the pair of series diodes must be much greater than the signal current. If this condition is not satisfied, the differential output currents produced at the collectors of the transistors become distorted. Further, in such circuit, the constant current cannot effectively be increased to satisfy the condition where the constant current is much greater than the signal current.
Another circuit is attempted in our copending application Ser. No. 83,858, which has matured into U.S. Pat. No. 4,292,597 to solve the problem in the above-identified circuit and, as such, produces no distorted output signals regardless of the relationship of the constant current and signal current and can be used with a relatively low voltage source. In such circuit, a constant current flows through a first path of two diodes connected in series and through a second path comprised of a diode and the collector-emitter path of a transistor connected in series. The signal current is supplied to the connection point between the two diodes of the first path, to the base of the transistor in the second path and to a second transistor. The collector of the first transistor is also connected to the base of a third transistor and differential output currents are produced at the collectors of the second and third transistors. In this circuit, the output currents at the collectors of the second and third transistors are not distorted, regardless of the relationship between the constant current and the signal current. Further, since this circuit does not require a bias circuit, the construction of the circuit is relatively simple and requires only a low voltage source. However, the differential output currents are affected by the base current of the first transistor and the base currents of the diodes when the latter are constructed from transistors having the collectors connected to the respective bases thereof. This means that the differential output currents will be affected by an undesirable off-set current which is a function of such base currents. This is also the case where the above circuit is modified so that a bias voltage source is connected between the first and second signal paths and ground and a constant current source is connected between the emitters of the second and third transistors and ground.